In conventional multi-phase fluid pumping systems, one or a plurality of system parameters are normally used to control one or a plurality of variable system parameters in order to keep the pump within a permissible operating region. The system parameters may, for example, comprise a parameter indicative of the differential pressure across the pump, e.g. the pump suction pressure, and the variable system parameters may, for example, comprise the rotational speed of the pump and/or the flow of fluid through a feed-back conduit leading from the discharge side to the suction side of the pump.
The operational range of a pump is generally illustrated in a DP-Q diagram (cf. FIG. 1). In the DP-Q diagram, the differential pressure over the pump is mapped against the volumetric flow through the pump, and the permissible operating region within the DP-Q diagram is identified. The border between the permissible operating region and an impermissible operating region is defined by the so called pump limit characteristics curve. Under normal conditions, the pump is operated only in the permissible operating region. However, if the pump enters the impermissible region, a pumping instability, or surge, may occur, in which case the pump may be subjected to a possible failure.
During operation of the system, the differential pressure across the pump and the flow of fluid through the pump may be monitored. If the monitored operating point approaches the pump limit characteristics curve, the rotational speed of the pump may be adjusted such that the pump is kept within the permissible operating region.
US 2002/0162402 A1 discloses a method for determining the flow rate through a pump based on a plurality of known speed and torque values. According to the method, characterising flow rate/torque information for the pump is retained and used to determine fluid flow rate at measured, non-characterized, speed and torque values. In order to establish the flow rate, the motor torque and the motor speed are measured and the corresponding flow rate value is looked-up in the retained flow rate/torque information.
However, in hydrocarbon fluid pumping applications, the gas volume fraction (GVF) and/or the density of the fluid may change quickly, e.g. due to gas and/or liquid slugs in the system. On the other hand, the differential pressure requirements across the pump will normally change relatively slowly due to slow changes in the production profile. With large volumes of compressible fluid upstream and downstream of the pump, and assuming that slug lengths are shorter than the lengths of the flow lines, the differential pressure requirement will be fairly constant, even if the pump sees density variations. As a consequence, a conventional multi-phase fluid pumping system using the differential pressure across the pump as a main parameter to control the system may not be fast enough to prevent the pump from entering the impermissible operating region.
The present invention addresses this problem, and an object of the invention is to provide a system for pumping a fluid and a method of operating the same which can react quickly to a change in the gas volume fraction and/or the density of the fluid.